As say it's name, emit particles from a geometry. For further informations about particles systems, go check this part of the doc :
Simulate a particle system
This modifier can be added over an existing particle system to make some trails. But this can also be done with
Emit a number of particles from another particle
. Go check the
Defines the behaviour of particles
part of the doc.
Beware that number of emitted particles represent the total of emitted particles. If you put this modifiers with a rate of 30 particles/seconds over o system containing millions of particles, you'll get 30 particles/second as a result.
Check this quick tutorial to see a little bit an extensive use of this modifier. This tutorial was made on an earlier version of Smode so the "place" modifier you see in use is the actual
Change every vertex position using scale, rotate and translate transformations
Particles simulation speed can be adjust through their
Playing parameters, position and length of the Function Cue